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Change-Id: I6bb44232b8878b1ef361149d7f99b2eafb8d9714
PaymentMethod utility class

The immediate gain is that utm_source payment method will be
constrained to sane values.  The payment method component will be
the name of the payment submethod "group", with an "r" prepended
for recurring donations.

There are also two long-term goals which are supported by this patch:
* Selecting the gateway by desired payment method
* Unique and meaningful designation for each payment method

Change-Id: I34602faa9d8cca99132aa3b8067247e0f0a58b9e